Section 7. Optional POP3 Commands

The POP3 commands discussed above must be supported by all minimal
   implementations of POP3 servers.
   The optional POP3 commands described below permit a POP3 client
   greater freedom in message handling, while preserving a simple POP3
   server implementation.
      NOTE: This memo STRONGLY encourages implementations to support
      these commands in lieu of developing augmented drop and scan
      listings.  In short, the philosophy of this memo is to put
      intelligence in the part of the POP3 client and not the POP3
      server.
      TOP msg n
         Arguments:
             a message-number (required) which may NOT refer to to a
             message marked as deleted, and a non-negative number
             of lines (required)
         Restrictions:
             may only be given in the TRANSACTION state
         Discussion:
             If the POP3 server issues a positive response, then the
             response given is multi-line.  After the initial +OK, the
             POP3 server sends the headers of the message, the blank
             line separating the headers from the body, and then the
             number of lines of the indicated message's body, being
             careful to byte-stuff the termination character (as with
             all multi-line responses).
             Note that if the number of lines requested by the POP3
             client is greater than than the number of lines in the
             body, then the POP3 server sends the entire message.
         Possible Responses:
             +OK top of message follows
             -ERR no such message
         Examples:
             C: TOP 1 10
             S: +OK
             S: <the POP3 server sends the headers of the
                message, a blank line, and the first 10 lines
                of the body of the message>
             S: .
                ...
             C: TOP 100 3
             S: -ERR no such message
      UIDL [msg]
      Arguments:
          a message-number (optional), which, if present, may NOT
          refer to a message marked as deleted
      Restrictions:
          may only be given in the TRANSACTION state.
      Discussion:
          If an argument was given and the POP3 server issues a positive
          response with a line containing information for that message.
          This line is called a "unique-id listing" for that message.
          If no argument was given and the POP3 server issues a positive
          response, then the response given is multi-line.  After the
          initial +OK, for each message in the maildrop, the POP3 server
          responds with a line containing information for that message.
          This line is called a "unique-id listing" for that message.
          In order to simplify parsing, all POP3 servers are required to
          use a certain format for unique-id listings.  A unique-id
          listing consists of the message-number of the message,
          followed by a single space and the unique-id of the message.
          No information follows the unique-id in the unique-id listing.
          The unique-id of a message is an arbitrary server-determined
          string, consisting of one to 70 characters in the range 0x21
          to 0x7E, which uniquely identifies a message within a
          maildrop and which persists across sessions.  This
          persistence is required even if a session ends without
          entering the UPDATE state.  The server should never reuse an
          unique-id in a given maildrop, for as long as the entity
          using the unique-id exists.
          Note that messages marked as deleted are not listed.
          While it is generally preferable for server implementations
          to store arbitrarily assigned unique-ids in the maildrop,
          this specification is intended to permit unique-ids to be
          calculated as a hash of the message.  Clients should be able
          to handle a situation where two identical copies of a
          message in a maildrop have the same unique-id.
      Possible Responses:
          +OK unique-id listing follows
          -ERR no such message
      Examples:
          C: UIDL
          S: +OK
          S: 1 whqtswO00WBw418f9t5JxYwZ
          S: 2 QhdPYR:00WBw1Ph7x7
          S: .
             ...
          C: UIDL 2
          S: +OK 2 QhdPYR:00WBw1Ph7x7
             ...
          C: UIDL 3
          S: -ERR no such message, only 2 messages in maildrop
      USER name
         Arguments:
             a string identifying a mailbox (required), which is of
             significance ONLY to the server
         Restrictions:
             may only be given in the AUTHORIZATION state after the POP3
             greeting or after an unsuccessful USER or PASS command
         Discussion:
             To authenticate using the USER and PASS command
             combination, the client must first issue the USER
             command.  If the POP3 server responds with a positive
             status indicator ("+OK"), then the client may issue
             either the PASS command to complete the authentication,
             or the QUIT command to terminate the POP3 session.  If
             the POP3 server responds with a negative status indicator
             ("-ERR") to the USER command, then the client may either
             issue a new authentication command or may issue the QUIT
             command.
             The server may return a positive response even though no
             such mailbox exists.  The server may return a negative
             response if mailbox exists, but does not permit plaintext
             password authentication.
         Possible Responses:
             +OK name is a valid mailbox
             -ERR never heard of mailbox name
         Examples:
             C: USER frated
             S: -ERR sorry, no mailbox for frated here
                ...
             C: USER mrose
             S: +OK mrose is a real hoopy frood
      PASS string
         Arguments:
             a server/mailbox-specific password (required)
         Restrictions:
             may only be given in the AUTHORIZATION state immediately
             after a successful USER command
         Discussion:
             When the client issues the PASS command, the POP3 server
             uses the argument pair from the USER and PASS commands to
             determine if the client should be given access to the
             appropriate maildrop.
             Since the PASS command has exactly one argument, a POP3
             server may treat spaces in the argument as part of the
             password, instead of as argument separators.
         Possible Responses:
             +OK maildrop locked and ready
             -ERR invalid password
             -ERR unable to lock maildrop
         Examples:
             C: USER mrose
             S: +OK mrose is a real hoopy frood
             C: PASS secret
             S: -ERR maildrop already locked
               ...
             C: USER mrose
             S: +OK mrose is a real hoopy frood
             C: PASS secret
             S: +OK mrose's maildrop has 2 messages (320 octets)
      APOP name digest
         Arguments:
             a string identifying a mailbox and a MD5 digest string
             (both required)
         Restrictions:
             may only be given in the AUTHORIZATION state after the POP3
             greeting or after an unsuccessful USER or PASS command
         Discussion:
             Normally, each POP3 session starts with a USER/PASS
             exchange.  This results in a server/user-id specific
             password being sent in the clear on the network.  For
             intermittent use of POP3, this may not introduce a sizable
             risk.  However, many POP3 client implementations connect to
             the POP3 server on a regular basis -- to check for new
             mail.  Further the interval of session initiation may be on
             the order of five minutes.  Hence, the risk of password
             capture is greatly enhanced.
             An alternate method of authentication is required which
             provides for both origin authentication and replay
             protection, but which does not involve sending a password
             in the clear over the network.  The APOP command provides
             this functionality.
             A POP3 server which implements the APOP command will
             include a timestamp in its banner greeting.  The syntax of
             the timestamp corresponds to the `msg-id' in [RFC822], and
             MUST be different each time the POP3 server issues a banner
             greeting.  For example, on a UNIX implementation in which a
             separate UNIX process is used for each instance of a POP3
             server, the syntax of the timestamp might be:
                <process-ID.clock@hostname>
             where `process-ID' is the decimal value of the process's
             PID, clock is the decimal value of the system clock, and
             hostname is the fully-qualified domain-name corresponding
             to the host where the POP3 server is running.
             The POP3 client makes note of this timestamp, and then
             issues the APOP command.  The `name' parameter has
             identical semantics to the `name' parameter of the USER
             command. The `digest' parameter is calculated by applying
             the MD5 algorithm [RFC1321] to a string consisting of the
             timestamp (including angle-brackets) followed by a shared
             secret.  This shared secret is a string known only to the
             POP3 client and server.  Great care should be taken to
             prevent unauthorized disclosure of the secret, as knowledge
             of the secret will allow any entity to successfully
             masquerade as the named user.  The `digest' parameter
             itself is a 16-octet value which is sent in hexadecimal
             format, using lower-case ASCII characters.
             When the POP3 server receives the APOP command, it verifies
             the digest provided.  If the digest is correct, the POP3
             server issues a positive response, and the POP3 session
             enters the TRANSACTION state.  Otherwise, a negative
             response is issued and the POP3 session remains in the
             AUTHORIZATION state.
             Note that as the length of the shared secret increases, so
             does the difficulty of deriving it.  As such, shared
             secrets should be long strings (considerably longer than
             the 8-character example shown below).
         Possible Responses:
             +OK maildrop locked and ready
             -ERR permission denied
         Examples:
             S: +OK POP3 server ready <1896.697170952@dbc.mtview.ca.us>
             C: APOP mrose c4c9334bac560ecc979e58001b3e22fb
             S: +OK maildrop has 1 message (369 octets)
             In this example, the shared  secret  is  the  string  `tan-
             staaf'.  Hence, the MD5 algorithm is applied to the string
                <1896.697170952@dbc.mtview.ca.us>tanstaaf
             which produces a digest value of
                c4c9334bac560ecc979e58001b3e22fb